Chemistry Everyday for Everyone
Gas Experiments with Plastic Soda Bottles Patrick Kavanah 7 Tappan Drive, Monroe, NY 10950 Arden P. Zipp Department of Chemistry, SUNY College at Cortland, Cortland, NY 13045
Apparatus
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Mass of Bottle / g
Most chemistry courses include one or more experiments on the behavior of gases. These may vary from simple demonstrations that gases have mass and decrease in volume when an external pressure is applied to the relatively sophisticated task of determining the molar mass of an unknown gas. We have developed an inexpensive device that can be used to carry out a range of such activities. It consists of a plastic soda bottle with a plastic cap modified as described below.
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A 3/8′′ hole is drilled through a plastic soda bottle cap and its blue plastic liner (from the inside) and fitted with an automobile tire valve.2 (This assembly may be sealed with glue or cement, although we have not found this to be necessary.) The cap is screwed on a clean, dry bottle (375 mL to 3 L) and a hand bicycle pump is used to fill the bottle with air to a pressure of about 40 psi (measured with a tire gauge). The mass of the bottle is obtained on a top-loading balance and determined again after a half hour. If the two masses are the same, the cap can be assumed to be air-tight and suitable for experimentation. (Some bottles and caps have maintained their mass and pressure for several months!) Tape may be applied to the outside of the bottle as a safety precaution, although we have not had a bottle explode even at 100 psi.
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Figure 1. Mass of 1.5-L bottle vs pressure.
Molar Mass of Air (or Other Gases) The mass of the “evacuated” bottle can be used along with its volume3 and the mass at any other pressure to calculate the molar mass of air by means of the equation 4
MM =
Mass of Air On the simplest level, as for elementary school classes, this apparatus can be used to show that air has mass by weighing a bottle alone and then pressurizing it with a tire pump and reweighing it. On a more advanced level, a series of mass–pressure readings can be used in a graphing exercise to find the mass or density of air at different pressures. In this case, the bottle is inflated to a pressure of about 40 psi, the pressure is measured, and the bottle is weighed. A small amount of air is released by depressing the center of the valve briefly and the pressure and mass are measured again. This process is repeated until several mass–pressure values have been obtained. A plot of these values can be interpolated or extrapolated to yield other masses, including that of the “evacuated” bottle. Because gauge readings give the pressure relative to atmospheric pressure, the actual pressure in the bottle at any gauge pressure equals the reading on the gauge plus one atmosphere (14.7 psi). Thus, the measured gauge pressures (in psi) can be corrected to actual pressures by adding 14.7. The mass of the bottle is plotted against the actual pressure and extrapolated to zero pressure to obtain the mass of the “evacuated” bottle, as shown below. (It should be pointed out that this method of determining the mass of an “evacuated” bottle does not require potentially expensive glass vessels and vacuum pumps to evacuate them or high capacity balances on which to weigh them.)
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Corrected Pressure / psi
gRT PV
A sample calculation from the results with a 1.5-L (1.56 L, actual volume) bottle follows (mass of “evacuated” bottle 68.62 g, mass of bottle at room pressure 70.36 g, room pressure and temperature 738.7 mm Hg and 22˚ C):
MM =
70.36 – 68.62 × 0.0821 × 295 738.7/760 × 1.56
= 27.8
MM = 27.8 g. (MM found from the density of moist air under these conditions is 28.0 g.) Once the mass of the “evacuated” bottle has been determined, the molar masses of other gases can be obtained using the ideal gas equation by filling the bottle with the particular gas by upward or downward displacement of air, as appropriate. (The pressure of the gas in the bottle equals the ambient atmospheric pressure at that time.) This technique represents a major improvement over experiments that employ flexible containers such as plastic bags and require corrections for the buoyancy of air, because students often find such corrections confusing. Other Activities The effect of pressure on volume can be investigated by placing a sealed5 syringe about half full of air in a bottle and pressurizing the bottle (1). The volume of air in the syringe
JChemEd.chem.wisc.edu • Vol. 75 No. 11 November 1998 • Journal of Chemical Education
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Chemistry Everyday for Everyone
is read at different pressures by releasing small amounts of air as described above. This technique is more quantitative than stacking textbooks on the plunger of a half-filled sealed syringe and allows the P–V relationship to be studied over a wider pressure range than most other methods. Qualitatively, the pressure–volume relationship can be illustrated by placing marshmallows in a bottle and crushing them under high pressure.6 (This is the opposite of expanding marshmallows under vacuum.) The bottle can be used as a cloud chamber by adding a small amount of water, pressurizing the bottle, and then releasing the pressure quickly. Heating (cooling) effects can be demonstrated during pressurizing (depressurizing).6 Notes 1. The size of the hole required will vary with that of the tire valve used. It is important that the size of the hole be such that the
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bottle cap fits snugly between the outer shoulder and the inner flange of the valve (just as the valve would fit a wheel). 2. Valve stems can be purchased relatively cheaply at auto parts stores or used ones can be obtained free for the asking from garages or tire repair shops. 3. The bottle volume may be found either by measuring the volume of water needed to fill it or by using the mass of air in the bottle at 1 atm (zero gauge) pressure and the density of air under laboratory conditions. 4. P for any pressure other than atmospheric is the corrected pressure (i.e., gauge pressure plus 1 atm). 5. After drawing the desired quantity of air into the syringe the syringe can be sealed either by attaching the plastic cap snugly or by attaching a needle and inserting this into a rubber stopper. 6. These two applications were suggested by a reviewer who also recommended placing temperature-sensitive liquid crystals on the bottle’s surface to show temperature effects more clearly.
Literature Cited 1. Delfiner, A. Chem Club News 1995, Nov., 1.
Journal of Chemical Education • Vol. 75 No. 11 November 1998 • JChemEd.chem.wisc.edu
